In the cooling of cryogenic devices, e.g. superconductive magnets, cable systems and the like, a recirculated coolant is brought to a low temperature in one or more stages and, in the last state, is partially expanded and fed to the object, preferably after passing in indirect heat exchange with the coolant fluid in a separator-evaporator. The separator-evaporator contains a body or bath of liquified coolant, e.g., helium, which may be partially evaporated in the indirect heat exchange with the coolant fed to the object. The cooling fluid, after traversing the object, may be further expanded to produce a liquid-gas phase mixture which is passed into the separator-evaporator so that the gas phase can be recirculated while the liquid phase is accumulated.
In general, the expansion of the fluid prior to its passage through the separator-evaporator in indirect heat exchange and into the object to be cooled takes place through a conventional throttle valve.
Such systems are known for helium circulation cycles in the cooling of superconductive magnets and may include one or more precooling stages in which the cooling fluid, previously compressed, is cooled by heat exchange and by subsequent expansion. Generally there is at least one expansion stage ahead of the final heat exchanger which, as described, may be a separator-evaporator containing a bath of the liquefied coolant so that the expanded stream is supercooled in indirect heat exchange with the liquid bath and then supplied to the object. The gas phase leaving the separator-evaporator may be used for heat exchange cooling of the compressed cooling fluid in one or more heat exchangers separated by expansion stages and is ultimately compressed to be recirculated to the stream flowing to the object.
The cooling effectiveness at the object to be cooled is proportional to the enthalpy difference across the inlet and outlet of the object multiplied by the mass flow of the coolant through the object. For superconductive magnets, to coolant through the object. For superconductive magnets, to maintain the superconductive state against the increase in enthalpy of the coolant, the temperature difference across the cooling object must be maintained as small as possible so that, for a given temperature and type of coolant, the desired result can only be maintained by increasing the mass flow of the coolant through the object.
However, in conventional systems this mass flow is established by the maximum throughput at the warm end of the last stage of the cooling cycle. It should be apparent that dimensioning the last stage of the coolant cycle to accommodate an initially large mass flow of coolant requires, in conventional arrangements, a corresponding increase in the dimensions of the capacities of the previous stages. This results in unnecessary both for operating energy and construction.